In addition to that, I am wondering if it would be possible to document what all the lines setting up Postfix are doing. There's a good chunk of commands that are fed to it without explaining what they are for.

the Howtos a prepared for the installation of the OpenSource ISP Controlpanel ISPConfig. ISPConfig configures the document types on a per site / domain basis. If they are not commented out, ISPConfig will not be able to turn them on in the vHosts.